Condensing system boiler options in an S-plan?

[ms1012]

I've added up my radiator output, it's roughly 12 kW (suggesting the rads are perhaps slightly undersized for the house).

It seems madness to recommend a 30kW boiler, I guess they're basing it on the current boiler being a 28kW unit.

I've asked the person supplying the Vaillant quote about pros/cons of 18/24/30 kW versions. I've also identified a local installer of Viessmann and will get a quote there too.

Thanks for all the advice so far, it's certainly been an interesting learning experience.

 

[Agile]

It seems madness to recommend a 30kW boiler, I guess they're basing it on the current boiler being a 28kW unit.

I would say that they are mad and totally unable to calculate the heat requirements or understand anything about boiler efficiency or operation.

So they just copy the last one!

Tony

 

[Agile]

I 'assumed' the efficiencies were gained by not having to heat the water that high in the first place... i.e. by modulating the boiler "appropriately for the conditions" so it could maintain the room/tank temp using lower heat output) rather than just cycling on/off and modulating by return water temp alone.

The same power is required to heat the house and it makes no difference if thats a lot of flow at 30° or a smaller flow at 80°.

Modulation is by adjustment of the power input to produce the required output flow temperature.

All boilers were on/off modulation until about 1980 when modulating boilers became more popular.

There is nothing wrong with on/off modulation and thats the basis of DACs and switch mode power supplies, high power amplifiers and inverters etc.

Tony

 

[ALEC1]

The same power is required to heat the house and it makes no difference if thats a lot of flow at 30° or a smaller flow at 80°.

This should read The same amount of heat is required not power!


If the heat is gained at higher efficiency through condensing technology less fuel will be used, how much is debatable!.

Its forcing the heat generator to be efficient thats important, and thats where controls are required to make the boiler work only as appropriate...

I'd take 1kw per rad and 1.5kws for standing losses....and select a boiler that modulates low...the controls as mentioned above will take advantage of the kw output and settle into a highly efficient mode...

 

[D_Hailsham]

Do you know much about how the VRT392 works? From what I have read, I *assume* that it maintains the appropiate flow temperature to achieve / maintain the set room (or DHW) temperature, based on the current indoor (or tank) temp!? i.e. In a similar way that WC does for external temp?

I didn't know the answer, so I did some digging and eventually found the answer in the Installation Manual, page 8 (Acrobat page 28 of 32) - "Room temperature control strategy". The options are: Thermostat or Modulating, which are described as:

Thermostat - on/off control;
Modulating control - the boiler flow temperature is controlled depending on the actual and target room temperature.

So you can have either - a plain on/off control, which maintains the water temperature constant; or a modulating control which regulates the water temperature according to requirements.

 

[D_Hailsham]

A typical condensing boiler is 12% more efficient when fully condensing but that requires a return temperature below 40C.

12% ?? Where do you get that idea from? An example would be helpful.

 

[D_Hailsham]

The same power [heat] is required to heat the house and it makes no difference if that's a lot of flow at 30° or a [/b]smaller flow[/b] at 80°.

What are you going on about?

The flow has nothing to do with the actual temperature but with the temperature differential. So if the temp diff is 20°C, the flow will be exactly the same for 80°/60° as it will be for 50°/30°.

The only time flow rate changes is if the differential changes, so the flow for a 20°C differential is approximately half that for an 11°C differential. This means the pump has less work to do, and pipes can be smaller. The only disadvantage of a 20°C differential is that radiator output is reduced by about 20%.

 

[UpgradeME]

Thermostat - on/off control;
Modulating control - the boiler flow temperature is controlled depending on the actual and target room temperature.

So you can have either - a plain on/off control, which maintains the water temperature constant; or a modulating control which regulates the water temperature according to requirements.

I did read that previously... I guess I wasn't sure on how 'intelligent' the control is though? i.e. Would it perform any 'intelligent' calc's to determine / keep the flow temp 'appropriate' like a WC control?

Do you have any idea what the 'preset value' "two-point" means for C9? (I guess it would make more sense if they showed you available options!? )

 

[Agile]

The same power [heat] is required to heat the house and it makes no difference if that's a lot of flow at 30° or a [/b]smaller flow[/b] at 80°.

What are you going on about?

The flow has nothing to do with the actual temperature but with the temperature differential. So if the temp diff is 20°C, the flow will be exactly the same for 80°/60° as it will be for 50°/30°.

The only time flow rate changes is if the differential changes, so the flow for a 20°C differential is approximately half that for an 11°C differential. This means the pump has less work to do, and pipes can be smaller. The only disadvantage of a 20°C differential is that radiator output is reduced by about 20%.

I am afraid that you are missing the point and adding to the confusion.

Firstly heat is a form of power. Thats why it can be measured in kWh regardless of being electricity or heat. Electric power easily converts to heat. Not very efficiently the other way round.

In a heating system you can heat a house with UFH at 30 C but you have to have a greater flow and a low differential. With rads you can have a higher differential and a lower flow.

The 12% increase in efficiency is the improvement you get if you can extract ALL the latent heat of vapourisation from the water in the flue gases of a gas boiler.

I am surprised that you have not found the 12% during your research. Its prominently taught on the energy efficiency course heating engineers should have been on.

Its well explained here:-

http://www.kayanson.co.uk/technical_spec8.htm

Tony

 

[D_Hailsham]

I guess I wasn't sure on how 'intelligent' the control is though? i.e. Would it perform any 'intelligent' calc's to determine / keep the flow temp 'appropriate' like a WC control?

No idea how intelligent it is. But from the description I quoted it seems to regulate the flow temperature according how much heat is required. Presumably it measures the rate of increase in the room temperature when the house warms up and the overshoot/undershoot when the set temperature is reached; similar to a TPI thermostat.

Do you have any idea what the 'preset value' "two-point" means for C9?

That also had me mystified until I realized it could be a mistranslation from the German original. So I went to Vaillant's German website and looked at the original version of the manual. "Two point" means "ON-OFF". So the default setting is as an ON/OFF thermostat.

"Selection possibility" in the column heading means "Options".
"Step width" means the interval between settings. So a step width of 1 for C1 means that you can adjust the temperature in 1 degree steps.

 

[D_Hailsham]

I am afraid that you are missing the point and adding to the confusion.

I don't think so.

Firstly heat is a form of power. That's why it can be measured in kWh regardless of being electricity or heat.

I know that, but not all readers would realize that they are synonymous.

Electric power easily converts to heat. Not very efficiently the other way round.

Which is why electricity costs about three times as much as gas!

In a heating system you can heat a house with UFH at 30 C but you have to have a greater flow and a low differential. With rads you can have a higher differential and a lower flow.

Exactly what I said. In any case you have brought in another topic - UFH versus rads - which has not been mentioned before.

The 12% increase in efficiency is the improvement you get if you can extract ALL the latent heat of vaporisation from the water in the flue gases of a gas boiler.

So you are talking about net efficiency, not gross.

Its well explained here:-

http://www.kayanson.co.uk/technical_spec8.htm[/QUOTE]
But that is comparing the difference between an old conventional boiler and a modern condensing one in condensing mode. The conversation in this topic has been about the difference in efficiency of a modern condensing boiler when it is in condensing mode compared to when it is not in condensing mode.

As a random example: the Baxi Bermuda 401 produces 11.7kW output for 15.9kW input - an efficiency of 73.6%. The Baxi Solo HE 12 produces 11.82kW for 13.34kW input when it is not condensing, which is 88.6% efficiency. So the improvement in efficiency is 15% even without taking condensing into account.

When the boiler is condensing (40°C mean temperature) the output is 12.81kW for 13.34kW input; an efficiency of 96.0%. So condensing, in this case, increases the efficiency by 7.4%.

 

[UpgradeME]

As a random example: the Baxi Bermuda 401 produces 11.7kW output for 15.9kW input - an efficiency of 73.6%. The Baxi Solo HE 12 produces 11.82kW for 13.34kW input when it is not condensing, which is 88.6% efficiency. So the improvement in efficiency is 15% even without taking condensing into account.

When the boiler is condensing (40°C mean temperature) the output is 12.81kW for 13.34kW input; an efficiency of 96.0%. So condensing, in this case, increases the efficiency by 7.4%.

Interesting! - Where did you find those figures? Do you have the condensing vs non-condensing figures for the Vaillant boilers? (ecoTEC 418 in my case ) or did I miss them in the Vaillant manuals!?

 

[Agile]

The 12% is the additional heat that can be extracted by recovering the latent heat of vapourisation.

So a condensing boiler will produce UP to 12% improvement when its condensing. The exact figure will depend on the return temperature and the design of the boiler.

The point you are demonstrating, but not explaining, is that condensing boilers are more efficient even when not condensing. Thats because their heat exchangers are designed to extract more heat from the flue gases.

Non condensing boiler have to be designed to NOT take too much heat out of the combustion gasses so that the flue stays above 150C just so they do NOT condense and corrode the flue and heat exchanger.

A condensing boiler is designed to resist corrosion and so can operate with flue temperatures below 90C making them more efficient even when not condensing below 55C.

A minor point but heat and power are not synonymous if you interpret that word as meaning being the same thing. Heat is a form of power. A chicken is a bird but a bird is not always a chicken.

Tony

 

[D_Hailsham]

Interesting! - Where did you find those figures?

From the Baxi Group Installation Manuals

Do you have the condensing vs non-condensing figures for the Vaillant boilers? (ecoTEC 418 in my case ) or did I miss them in the Vaillant manuals!?

Yes, they are in the installation Manual on page 8. But you have to be very careful as it shows the net input.

For the 418 you have:

Net input = 18.9kW
Output at 80/60 (non condensing) 18.6kW
Output at 50/30 (condensing) 20kW

This gives efficiencies of 98.5% and 105.8%! (Perpetual motion comes to mind!)

Further down it gives the Gross CV of the gas as 37.8MJ/m³ and the consumption as 2m³/hour. Simple maths shows that this is equivalent to a gross input of 21kW. So the gross efficiencies of the boiler are: 88.6% and 95.2%.

Net values assume that none of the water vapour is condensed, so no energy released. Gross values assumes that all the water vapour is condensed, so all the energy in the water vapour is released.

Continental manufacturers use net values as it shows how much more you get when the boiler condenses. The UK uses gross figures as that is the method used by Sedbuk and the Standard Assessment Procedures (SAP 2005 and SAP 2009.

 

[ALEC1]

Continental manufacturers use net values as it shows how much more you get when the boiler condenses. The UK uses gross figures as that is the method used by Sedbuk and the Standard Assessment Procedures (SAP 2005 and SAP 2009

Precisely because in the UK we dont have to use controls that force the flow and return temperature down...

just commissioned a boiler on UFH and weather comped 23c flow, 15c return 18c flue gas temperature....

Thank you agile and d_hailsham....very informative and well explained (at last!)